For secondary batteries, miniaturization is essential because of miniaturization of mobile devices and limitation on installation space. To this end, attention has been paid to lithium-ion batteries with a high energy density. Although metal cans have been hitherto used as a packaging material for lithium-ion batteries (hereinafter simply called “packaging material”), multi-layered films which are lightweight, high in heat radiation and low in costs have been in use.
However, lithium salts such as LiPF6, LiBF4 and the like are used as an electrolyte of lithium-ion batteries. These lithium salts generate hydrofluoric acid by hydrolysis. Hydrofluoric acid may sometimes cause corrosion on the metal surface of battery members and a lowering of laminate strength between adjacent layers of a packaging material formed of a multilayer film.
To avoid this, an aluminum foil layer is disposed inside of a packaging material made of a multilayer film so as to inhibit moisture infiltration into the battery from the surface of the packaging material. For instance, a known packaging material includes, as successively stacked, a substrate layer having a heat resistance/first adhesive layer/aluminum foil layer/anti-corrosion layer/second adhesive layer/sealant layer. The lithium-ion battery making use of the above packaging material is called an aluminum-laminated type lithium-ion battery.
For an aluminum-laminated type lithium-ion battery, there is known, for example, an embossed lithium-ion battery wherein part of a packaging material is formed with a recess by cold forming, and a battery body (including a cathode, a separator, an anode, an electrolytic solution, etc.) is accommodated in the recess, and a remaining portion of the packaging material is folded back and sealed along its marginal portion by heat-sealing. In recent years, a lithium-ion battery has been fabricated wherein two sheathing sheets to be bonded together are, respectively, formed with a recess for the purpose of increasing an energy density and thus, more battery bodies can be accommodated.
The energy density of a lithium-ion battery becomes higher as the recess formed by cold forming is made deeper. However, a deeper recess is more prone to causing pinholes or breakage in a packaging material at the time of the forming.
Accordingly, a biaxially stretched polyamide film having excellent formability has been in wide use as a substrate layer. Since the biaxially stretched polyamide film is not well resistant to an electrolytic solution, a laminate film wherein a biaxially stretched polyester film is laminated on the biaxially stretched polyamide film has been used as a substrate layer (e.g. Patent Literature 1).
Use of a film as a substrate layer whose rates of elongation in four directions of 0°, 45°, 90° and 135° relative to the MD direction (flow direction) are, respectively, not less than 80% has been proposed (Patent Literature 2).